The present invention relates to the art of forming structural components such as used in motor vehicles and more particularly to a method of forming a tubular blank into a structural component by use of high pressure inert gas.
The invention involves formation of tubular metal components of structural type as used in automotive support frame where a tubular blank is formed to match the shape defined by the inner surface of a shell or cavity. In accordance with the invention, the shell or cavity is in a low permeability cast support structure wherein induction heating coils are embedded for inductively heating the tubular blank preparatory to formation into the desired shape imparted by the shell or cavity. A similar technology has been developed by Boeing Company wherein a flat plate is formed against a contoured wall by gas pressure. This process is referred to as superplastic forming of the plate and is disclosed in Gregg U.S. Pat. No. 5,410,132, incorporated by reference herein. This patent illustrates a process whereby the temperature of the metal plate is increased to a superplastic temperature by induction heating conductors mounted in a ceramic, low permeability cast die surrounding the metal forming chamber defined between two dies. This gas pressure chamber includes one surface against which the plate is formed. The Boeing process, as disclosed in Gregg U.S. Pat. No. 5,410,132, utilizes induction heating coils for the purposes of heating the metal preparatory to forming against a shaped surface by using high pressure gas on one side of the plate. The extent to which the Boeing patent defines a ceramic die with embedded induction heating coils and the use of a high pressure inert gas for forming the metal sheet, the technology relates to the technology employed in the present invention. For that reason, details of the die induction heating coils and high pressure gas forming may not be repeated to understand the present invention. In Matsen U.S. Pat. No. 5,530,227, Boeing Company further illustrates more details about the die, induction heating coils in a cast die forming material and the dies used by Boeing Company for superplastic forming of a sheet metal plate. Matsen U.S. Pat. No. 5,530,227 is also incorporated by reference herein so that the details of the technology developed by Boeing Company and usable in the present invention need not be repeated. Hot metal gas forming of steel is generally described in a joint venture proposal to the National Institute of Standards and Technology on Mar. 18, 1998. The proposal is incorporated by reference herein as background information.
The present invention is primarily directed toward the production of structural components of the type used in the automotive field and it will be described with particular reference thereto; however, the invention is much broader and may be used for forming various structural components from tubular sheet metal blanks. In the past, such structural components were normally produced by stamping, forming and welding. In an effort to obtain complex shapes, such components have been formed by a hydroforming process wherein tubular blanks are provided from sheet steel material having specific initial strength and elongation. The tubular blank is cut to length and pre-bent or preformed into a shape approximating the shape of the finished structural component. The preformed tubular element is loaded into a two piece die closed in a hydraulic press typically having a closing pressure between about 3500-8000 tons. The exposed ends of the tubular blank are sealed and the tube is filled with a water and oil mixture. The internal pressure of the water and oil mixture is raised to a high level in the general neighborhood of 20,000-80,000 psi which pressurized liquid expands the tubular blank into the shape of a steel die cavity machine in two die members of a die set carried by the hydraulic press. The cavities of the two die members have the desired final shape for the structural component so that as the tubular blank is expanded into the cavity, the outer shape of the component captures the shape of the cavity. This process produces a relatively accurate complex outer shape for the structural component. To relieve the fluid pressure, holes are pierced into the formed structural component. Thereafter, the two die members are opened by the hydraulic press and the liquid is drained from the formed structural component. Secondary machinery operations, such as trimming and cutting mounting holes, is then performed to produce a desired component for final assembly. This process is gaining popularity because it forms the component from the inside so complex shapes are possible; however, the total cycle time for hydroforming is at least about 25-45 seconds. The equipment to direct high pressure liquid into the tubular blank is extremely large and expensive. In addition, the die members are expensive machined parts and have a short life. Hydroforming operations have a general limitation that they are used primarily for bending of the tubular blank, since the steel being formed is processed at ambient temperature which limits the maximum strain rate for the metal being formed. Pressure of the liquid used in the hydroforming must be extremely high to deform the relatively cold sheet metal of the tubular blank into simple configurations. Consequently, hydroforming is used primarily for bending and straightening tubular elements into the desired final shape. Even though there are process limitations in using hydroforming to make tubular structural components, a substantial technology field has developed around this process. In a feature of hydroforming, the sheet steel tubular blank is formed into desired shapes while additional material is forced axially into the die cavity so the wall thickness does not drastically decrease as the volume of a given cross section increases during the processing by high pressure liquid.
Hydroforming is the primary prior art constituting the background of the present invention. However, blow forming of plastic sheets has been used for years to produce high volume plastic containers using conventional steel die members. Of course, such die members used in plastic blow forming can not be used for forming steel. For that reason, hydroforming is used for metal, instead of blow forming as used in the plastics industry. The highly developed technology of hydroforming of steel tubes and blow forming of plastic sheets are background of the present invention, but are not economically usable for forming sheet steel tubular blanks into tubular structural components. In addition, these prior processes do not have the capability of controlling the metallurgical characteristics along the length of the tubular blank, as obtainable by the present invention.
Even though hydroforming of sheet steel and blow forming of plastic sheets are the basic background to the present invention, it has been found that certain features of the technology disclosed by Boeing Company for superplastic forming sheet metal plates by high pressure gas are used in practicing the invention. The Boeing Company process is not background information from the standpoint that it is not capable of forming a tubular metal blank into a structural tubular component and is not capable of controlling the metallurgical characteristics of the metal forming the structural tubular component. These are all advantages of the present invention.
The present invention provides a completely different type of technology which is dissimilar to hydroforming of steel and blow forming of plastic sheets. In accordance with the present invention, a tube is made from sheet metal formed by controlled rolling of the sheet. The sheet metal is formed into a tubular blank which is preheated using resistance electric heating and preformed to the desired axial profile. The preheated blank is placed into the shell or cavity of a specially constructed die set in which are embedded induction heating conductors or coils spaced axially along the cavity. The tubular blank has an open end or open ends which are plugged or sealed. The tubular blank is expanded by high inert gas at a pressure in the general range of 100-5,000 psi, but preferably in the range of 200-1000 psi. During expansion, the induction heating conductors or coils induce an A.C. voltage into the metal of the blank which cause I2R heating of the blank. Consequently, the blank can be rapidly expanded. The cavity or shell having the desired predetermined shape surrounds the expanding tube to impart, to the outer surface of the blank the shape of the shell or cavity. The structural element is then cooled at a controlled quenching rate to control the metallurgical characteristics to enhance the mechanical properties of the resulting structural components.
By using the present invention there is developed a new metal forming process technology that reduces the cost to process tubular structural components by at least 50% and reduces the time to build, and the cost to build, the forming die members by at least about 40%. By using structural components formed by the unique process of the present invention, the structural component is reduced in weight by approximately 20%. Although the inventive method involves the use of gas to expand the sheet metal tubular blank into the desired configuration for the structural element, the invention actually involves substantial improvements in this general process. In other words, the present invention is not merely the use of high pressure inert gas as a substitute for high pressure liquid used in hydroforming. One aspect of the invention involves the formation of a unique cavity or shell which is mounted in the die members of the die set opened and closed by the hydraulic press. The shells and die members are constructed so that the tubular blank being formed into the shape of the shell or cavity can be heated inductively along its length to control the heat of the tubular blank before and during the forming process. This can not be done in hydroforming. By using induction heating in the tools or die members, the heating conductors or coils can localize the heating along the length of the blank. The die set not only supports induction heating conductors, but also (a) supports the forces necessary to restrain the tubular blank being formed and (b) provides increased wear resistance. By using the present invention, the yield strength along the length of the resulting component or end product is varied by proper heating and cooling. This is particularly advantageous if extended deformation is required in producing the desired finished shape of the structural element.
By using the present invention, a tubular structural component can be formed having more detailed outer configurations than obtainable with hydroforming. Indeed, the invention obtains the result generally associated with blow forming plastic sheets, but for sheet metal components. This is accomplished by utilizing a unique and novel material from which the die member containing the forming cavity is constructed so induction heating along the length of the tubular blank can be varied. Consequently, the material utilized for the shape defining cavity or shell has low permeability and is rigid. It is supported in a cast low permeability material holding the forming cavity in two die members movable together by a hydraulic press. By making this type of die member, induction heating along the tubular blank can be varied so that subsequent cooling of specific portions of the structural component provides desired metallurgical characteristics. The end product does not need to have a uniform metallurgical characteristic associated with the total processing operation which is the result of the Boeing process. Such process uniformly heats the sheet and does not quench harden the sheet.
In accordance with the present invention there is provided a method of forming an elongated tubular metal blank into a tubular structural component having a predetermined outer configuration, wherein the method uses a shape imparting shell formed from a low permeability, rigid material. The shell is in the form of a first and second half shell each of which includes an inner surface defining the predetermined shape of the final structural component. The half shells have laterally spaced edges which define a parting plane between the two half shells when the half shells are brought together. The half shells form a total shell or cavity having an inner surface defining the shape to be imparted to the structural component as the blank is expanded into the cavity. One half shell is mounted in one die member and the other half shell is mounted in the other die member so the die set can be opened and closed to define the part forming cavity. By employing a rigid, hard material defining the shape to be imparted to the final part, the shell can be supported as a separate element in a cast non-magnetic material held in the framework of the dies. By utilizing a cast material, together with an inner shell or cavity engaging the workpiece itself, the properties of the shell are not dictated by the compressive force carrying capacity necessary for the cast material. Consequently, by using a cast material which is different from the rigid, hard material actually engaging the tubular member during the forming process, both of these materials can be optimized. Since the invention utilizes induction heating surrounding the shell, the material of the shell and the material supporting the shell are both low permeability to be generally transparent to the magnetic fields created by the conductors embedded in the cast material. To expand the blank, the open ends of the tubular blank are plugged while in one of the half shells in one of the die members. The other half shell is then positioned over the blank and held in position by pressure in the general range of 50-500 tons. Thereafter, the tubular blank is formed into the final shape by inductively heating axial portions of the blank. When spaced portions are heated axially spaced conductors adjacent the shell or cavity are used. The heating is done while the tubular blank is forced into the cavity to create the desired shape. Consequently, tube expansion is accompanied by forcing an inert gas, such as nitrogen or argon, at high pressure into the plugged blank until the blank conforms to at least a portion of the inner surface of the first and second half shells defining the shape cavity during and/or after the tube is inductively heated. By utilizing conductors spaced axially along the workpiece and embedded in the cast material around the shell, the metal of the blank is inductively heated to facilitate the forming operation caused by the expansion action of the internal gas pressure. By using the present invention, the total length of the tubular blank can be heated inductively and/or selected portions can be heated inductively. In practice, induction heating is normally accomplished to a greater extent where the primary formation or elongation is to be accomplished in practicing the invention. In this aspect of the invention, there is provided a unique formation two component die member. The inner component defines the shape and the outer component defines the compressive force absorbing mass. Thus, the two components of the member can be optimized. A better shape can be imparted to the workpiece and an inexpensive compressive force absorbing cast material can be used. This cast material is employed for embedding the induction heating conductors that inductively heat of the tubular blank during forming or prior to forming. Indeed, the induction heating can be before and/or during the gas forming operation.
In accordance with this aspect of the invention, the high hardness rigid material or shell is ceramic and preferably fused silicon. It is also possible to select material from the class consisting of silicon nitride, silicon carbide, beryllium oxide, boron oxide and zirconium. In the preferred embodiment the shell has a thickness of xe2x85x9c-⅝ inches and is cast from silicon nitride with or without sintering. Then a hard cutting tool type ceramic may be coated on the shaped surface. In another process for making the rigid hard shell, powder silica is compressed to 50%-70% and then the shape is machined into the block. Vacuum removes the air while nitrogen is used to penetrate. This gives a silicon nitride shell. It may be a block supported in the die member or a thin walled shell. As can be seen, the ceramic material used to construct the shape imparting shell is different than the inexpensive ceramic material forming the remainder of the die member, which material is merely a compressive force resistant material supported in a metal framework. The shell may be coated.
It is possible to select materials, such as oxides, i.e. refractory cements, glass ceramics, high strength ceramics (e.g. silicon nitride, silicon carbide, zirconium oxide etc.). These materials can be either monolithic, or with various forms of reinforcements (composites), such as ceramic particulate reinforced glass. As an example, in one process for making the rigid hard shell, powder silica is merely compressed by more than 60% of full density. In one process, a silica-based glass ceramic is melted, mixed with silicon carbide reinforcement and cast into the desired shell shape.
In accordance with another aspect of the present invention, the predetermined shape has an axial profile which may undulate. Thus, the final part may have curves and bends. It is within an aspect of the invention to preform tubular blank into this axial profile so the blank generally corresponds to the profile of the final part.
In accordance with another aspect of the invention, the tubular blank is resistance heated by passing an alternating current, or direct current, through the sheet metal of the blank preparatory to moving the hollow or tubular blank into the forming shell. Induction preheating is also used. Consequently, the total tubular blank is at an elevated temperature so that the induction heating of the blank merely raises the temperature beyond the preheated temperature of the blank.
In accordance with another aspect of the present invention, the induction heating is varied along the length of the tubular blank or over the locations of the flat hollow blanks whereby different locations are inductively heated to different temperatures, at different time intervals, to achieve optimal strain distribution control. Indeed, axial portions of the workpiece are inductively heated in different induction heating cycles dictated by the desired metallurgical characteristics and deformation amount at axial portions of the tubular blank. By changing the induction heating effect along the blank preparatory to forming, or during forming, the induction heating process is xe2x80x9ctunedxe2x80x9d with temperatures at different locations on the tubular blank. In this manner, the desired metallurgical characteristics and/or the optimum forming procedure is obtainable. The use of induction heating in this manner to selectively process portions of the tubular sheet metal blank distinguishes the present invention from any prior forming processes.
The use of induction heating to different degrees at various portions of the tubular blank allows thermal processing of the various portions differently. Variations in the induction heating along the length of the blank can be accomplished by a number of coils or conductors along the forming cavity. The heating cycle of selected portions is controlled by varying the frequency, the power, the distance of the conductors from the workpiece, the spacing between axially adjacent conductors and the induction heating cycle time. By changing one or more of these induction heating parameters, the tubular blank being formed has controlled heating along its length. The temperature is controlled. For steel, it is generally 1400xc2x0 F. to 1800xc2x0 F. Aluminum is heated to a lower temperature. The objective is to produce a specific temperature that creates the proper formability plasticity.
In accordance with another aspect of the present invention, the heated, formed tubular structural component is transferred into a quench station. In the quench station, the previously inductively heated structural component is liquid and/or air quenched along its length. In accordance with another aspect of the invention, the quenching action is also xe2x80x9ctunedxe2x80x9d along the length of the workpiece. By controlling the amount of heating during the forming process and the quenching time, flow rate and/or temperature of the liquid, metallurgical properties of the steel or aluminum forming the structural component is controlled at various portions along the length. By practicing the present invention, the tubular blank is inductively heated in a controlled fashion at various locations along the length of the blank. The resulting tubular structural component is then quenched in a controlled fashion to dictate the metallurgical characteristics along the various portions along the length of the structural component.
In accordance with a more limited aspect of the present invention, as the tubular blank is expanded into the shape of the shell or cavity carried by the two spaced die members, portions of the tubular workpiece outside of the die members is pushed into the cavity or shell to provide additional metal to prevent drastic reduction in the wall thickness when substantial expansion of the tubular blank is dictated by the desired final shape of the structural component. This procedure is a concept used in hydroforming of steel tubular blanks. In accordance with still a further aspect of the invention, the pressure of the forming inert gas within the tubular blank is sensed and controlled at the desired pressure. The gas pressure is controlled in the general range of 200-1000 psi which is sufficient to expand the inductively heated workpiece by using the method of the present invention. The gas pressure is controlled either by controlling the pressure introduced into the plugged tubular blank or, in the alternative, by venting the pressure from the blank.
In accordance with still a further aspect of the present invention, there is provided a die set for forming an elongated tubular steel blank into a tubular structural component. This die set comprises a shape imparting shell formed from a low permeability, rigid material. The shell has a thickness of xe2x85x9c-{fraction (58/8)} inches and is preferably formed from cast silicon nitride which is a hard cutting tool type ceramic. In practice, a non-sintered silicon nitride shell has a thin coating on the inner shaped surface of the shell formed by sputter deposed dense silicon nitride. Coatings of silicon carbide and titanium nitride have also been used. The hard shell is in the form of first and second half shells each of which includes an inner surface (preferably a coated surface) defining the predetermined shape, an outer support and mounting surface and spaced lateral edges which edges define the parting plane between the two half shells when the half shells are brought together. The halves form a total shell into which the tubular blank is expanded. The first die member has an upper side and a lower side and a non-magnetic support framework for carrying the first half shell mounted in the metal framework by a cast compressive force transmitting non-magnetic fill material. This fill material is preferably fused silica. The lateral spaced edges of the first half shell facing outwardly from the lower side of the first die member. In a like manner, the second die member has an upper side and a lower side and a non-magnetic support frame for carrying the second half shell mounted in the framework by a cast compression force transmitting non-magnetic fill material. The fill material is preferably fused silica. The laterally spaced edges of the second half shell facing outwardly from the upper side of the second die member. The first and second die members are moved together to capture the blank in the shape imparted shells formed by the hard, rigid shell halves. The two die members carry a shell formed from a hard, rigid material selected for the purposes of long die wear. The shell material is selected to maintain the desired shape of the shell for long periods of time. By using this type of die set, the induction heating conductors or coils are embedded within the cast fill material surrounding the shape imparting inner surface of the hard, rigid shell.
In accordance with another aspect of the present invention there is provided a method of forming an elongated tubular blank into a tubular component having a predetermined outer configuration. This method comprises plugging the open end or ends of the tubular blank, placing the plugged blank into a shell or cavity with an inner surface surrounding the blank and having a predetermined outer configuration, forming the tubular blank into the component by inductively heating along the length of the blank by axially spaced conductors surrounding the cavity, while forcing inert gas at a high pressure into the plugged blank until the blank conforms to at least a portion of the inner surface of the cavity or shell to form the desired final component. The inert gas is nitrogen or argon. The shell has a thickness in the general range of xe2x85x9c-⅝ inches and the metal being formed is steel and aluminum.
In accordance with still a further aspect of the invention, an elongated tubular blank is formed into a tubular component having a predetermined outer configuration. This method involves plugging the open ends of the tubular blank, placing the plugged blank into a shell or cavity with an inner surface surrounding the blank and having a predetermined shape, forming the tubular blank into the component by inductively heating axial portions along the length of the blank while forcing gas at high pressure into the blank until the blank conforms to at least a portion of the inner surface of a cavity and transferring the formed component to a quench station where the component is selectively quenched along its axial length.
The induction heating used in the present invention is varied along axial portions of the tubular blank and the quenching is also controlled along the axial length of the blank. In this manner, the forming operation is optimized and the metallurgical properties of the resulting structural component are optimized. Since the invention is a hot forming process, it provides a means to significantly improve material formability. Within the acceptable forming time (i.e. 15 seconds), or deformable speed (strain rate greater than 0.1 per second), the process achieves more than 100% uniform tensile elongation for several aluminum alloys, as compared to about 30% in cold forming processes. The hot metal gas forming provides enhanced formability, thus greatly enhances manufacturability of structural parts and offers increased design flexibility. Consequently, the process part has reduced weight, tooling costs and development time.
The primary object of the present invention is the provision of a method of forming a tubular metal blank into a tubular structural component, with the desired outer shape, which method controls the heating and metallurgical characteristics by controlled induction heating and controlled quenching.
A further object of the present invention is the provision of a method, as defined above, which method overcomes the disadvantages of hydroforming such as limited shapes, low die life and high equipment costs.
Still a further object of the present invention is the provision of a method, as defined above, which method has reduced the tooling cost, reduced process cycle time, and increased design flexibility.
Yet another object of the present invention is the provision of a method, as defined above, which method allows size or shape changes substantially over 10% of the original tube diameter without requiring secondary operations or material annealing operations between processing.
Still a further object of the present invention is the provision of a die set for practicing the method as defined above, which die set includes a shell or cavity formed from a hard, rigid ceramic supported by a non-magnetic cast fill material so the shell has long life and the fill material has high compressive force characteristics.
Another object of the present invention is the provision of a method, as defined above, which method involves expanding a tubular workpiece heated inductively by controlled heating cycles. Then selectively quenching the workpiece is used to control the metallurgical properties of the finished product using rapid quenching, arrested cooling or combinations thereof.
These and other objects and advantages will become apparent from the following description taken together with the accompanying drawing.